The invention relates generally to radiation therapy for in vivo treatment of malignant cells in humans and animals, and more particularly to the method and apparatus for selective application of the electron flux to tissue with minimal damage to healthy tissue.
Radiation therapy is commonly understood to comprehend the application of ionizing radiation to a body for the purpose of damaging malignant tissue. The typical example is the use of bremmstrahlung X-rays from an electron accelerator or the collimated ray photons from a radiaoactive source, such as 60Co. All tissue in the path of such a radiation beam are exposed to the effects of the ionizing radiation. These energetic photons are highly penetrating and may interact with the tissue through which they pass on a statistical basis. There is a probability for a particular interaction depending upon the photon energy and the nature of the material (tissue) through which they pass. The most likely interactions for electron energies greater than about 1 KeV, include the Compton effect, and above a photon energy of 1.2 MeV, pair production. These interactions may be regarded as mechanisms whereby energetic electrons (and, for pair production, also positrons) are produced at the site of the interaction. Often, lower energy photon(s) are also produced and these proceed further into the tissue, in somewhat different direction from the original photon direction, and the process proceeds, on a probabalistic basis for individual electron scattering events. That is, a photon flux emerges from the exit surface of the body which differs somewhat from the character of the photon flux on the entry surface. This simply reflects that the likelihood of interaction is small for photons transiting tissue in comparison with material of high atomic number, such as lead. The effect of that portion of the photon flux which does interact in the path directed through tissue is substantial due to the damage done to cells by the energetic electrons resulting from photon scattering and absorption processes. Such damage is suffered alike by healthy and malignant cells, but malignant cells are typically more sensitive to radiation than are healthy cells. Much effort has been directed to reducing the damage done to healthy tissue.
Direct utilization of electron fluxes is also known. Electrons are available from radioactive sources (-emitters) or as controllable energetic, focussed beams from accelerator sources. Radioactive sources present hazards in handling, and emission therefrom, is uncontrolled in direction, energy and intensity. In any use of direct electron treatment, it must be recognized that electrons lose energy in traversing any path through matter. In contrast with the interaction of photons in passing through matter, the likelihood is very high for interaction by electrons with the environment through which they pass. The mechanisms need not be discussed here other than to recognize that as a result of such interaction, the electron loses energy and momentum to its environs in collision with atoms, and it continues on a trajectory which differs in angle and energy (a scattering event) where the differences depend upon the initial energy of the electron and the nature of the environment (the specific character of atoms encountered in such trajectory).
In the prior art, there is described method and apparatus for directing an electron beam through a hollow needle to scatter from a metallic disk affixed to the distal end of the needle. As described, an electron beam of several MeV energy scatters from a foil of aluminum, titanium or heavier metal which may, in a typical case have a thickness in the range 0.002 to 0.038 inches. This thickness can be greater however, provided that it is less than the range of the incident electrons in the material being used. The scattered electrons then emerge to fill a treatment volume defined by the therapy. In practice, this arrangement provides a somewhat diffuse point source of electrons having an angular distribution which, in general, is peaked in the forward direction. The angular distribution may be made almost spherical through proper choice of incident energy, scattering material and material thickness. This is described further in U.S. Pat. No. 5,585,643 to the present inventor and incorporated herein by reference. This prior art was directed to treatment of subcutaneous tissue, principally through needle insertion procedures.
When irradiating the site of a malignancy directly with electron radiation, it is desirable to obtain a uniform treatment over the tissue to be treated. In the context of the present invention, a surgical procedure has usually been performed to excise diseased tissue and it is desired to treat a thin layer of surrounding tissue by irradiation. In practice, the dimensions of the tissue area under treatment is many times the dimensions of the typical needle (18 gauge) as described in the prior art.
The need remains for an improved uniform areal density of electrons having substantially uniform spectral properties for impinging the tissue under treatment, together with ability to treat such larger area uniformly and rapidly.
The present invention provides for directing an energetic electron beam through a hollow needle to scatter from a high Z material to produce a relatively spherical distribution of electrons as disclosed in the prior U.S. Pat. No. 5,585,643. In the present invention, the needle is surrounded by a scattering body (or sphere) of typically low atomic number. The scattering body is preferably a tissue equivalent material which provides for multiple scattering of electrons. These multiply scattered electrons emerge from the surface of the scattering body as a shell of electrons, having effectively absorbed the core of radiation from the quasi-point source of the prior art. The electron flux emerging from the surface of a (homogeneous) scattering body is substantially isotropic and homogeneous in energy. Inhomogeneities may be incorporated into the scattering body to obtain desired anisotropies and energy distributions.
In another aspect of the invention, the patient under treatment is supported in an electrically insulated manner and the charge deposited thereupon is measured through electrical connection from the patient through a charge integrator or a picoammeter to ground for charge integration, whereby the total radiation density encountered is quantitatively known and the dosage to any tissue treated is known from the measured dose distribution data.